One Belfast firm lost £10,000 in just one weekend while one in North Down was scammed to the tune of £3,000.

"It is the business that is solely responsible for the fraudulent call charges so it really can have a detrimental effect on any firm in Northern Ireland," warned David Beatty, from Rainbow Communications, which manages telecommunications for 5,000 companies in the UK and Ireland.

The service industry and retail outlets fell foul of the fraudsters within the past few weeks. The PSNI said it was aware of the crimes. A spokesman said: "We have received reports and are investigating."

Fixed line fraud can be done in a number of ways. In some cases, fraudsters gain access to a switchboard and sell other people the ability to make calls through the switchboard. This is known as Dial Through Draft or Direct Inward System Access Fraud.

There are also call selling scams where criminals take out a phone service and sell other people the ability to make calls through it or take out a phone service in a false name and leave a bad debt. Figures suggest global losses due to phone fraud amount to £9.9 billion a year.

Mr Beatty said: "This type of phone hacking is a real and serious concern for businesses in Northern Ireland. Fraudsters are making thousands of international or premium rate calls resulting in substantial costs at the expense of the unsuspecting company."

Businesses have been advised to take steps to thwart potential scams. They include barring unnecessary call destinations such as international or premium rate numbers; limiting after hours, weekend or holiday time calling; and ensuring that only essential personnel have remote access to systems.

A spokeswoman for Action Fraud said businesses should be vigilant and urged any company that falls victim to this type of crime to report it.